Many high value products made from copper, silver, gold, other noble metals and alloys thereof can be subject to soiling, dulling or tarnish formation due to atmospheric contaminants, environmental conditions or by handling and use. These metal objects can often form surface deposits of one or more metal oxides or metal sulfides that can be difficult to remove without reducing the appearance, quality or purity of the metal surface. Many mechanical cleaning methods cause scratching or marring of the surface. Chemical cleaners can visually change surface appearance. Cleaners and polishers are available, but involve substantial time and effort in application, are often difficult to use, tend to clean unevenly and fail often to remove all tarnish or prevent the reformation of an undesirable deposit on the surface.
Aqueous solutions are known in the art for the purpose of removing stains, corrosion or dirt from metal surfaces including, for example, Kendall, U.S. Pat. No. 3,997,361, disclosing a phosphoric acid, nitric acid combination in high concentration for tarnish removal. Warner et al., U.S. Pat. No. 3,640,736 and Warner, U.S. Pat. No. 3,846,139, teach organic thiol compositions that can be used for silver and copper tarnish removal. Combe, U.S. Pat. No. 3,619,962, teaches a liquid abrasive cleaner having a substantially basic pH and an abrasive to remove tarnish from silver and copper. Kolodny et al., U.S. Pat. No. 3,413,231, teach specific trivalent phosphorus compounds that impart tarnish resistance to the surface of subject metals. Similarly, Kroll et al., U.S. Pat. No. 3,330,672, relies on thiol compounds, amino compounds, typically quaternary amines, and organic surfactant materials for tarnish removal. Wassell, U.S. Pat. No. 2,393,866, teaches a metal tarnish remover comprising polyethylene oxide organic surfactant materials and other components in an aqueous solution for cleaning purposes. Bart, U.S. Pat. No. 1,947,180, teaches that silver can be made tarnish resistant using a coating of certain metals over the silver surface, typically added by an electroplating step. Other paste-like abrasive cleaning compositions are known that rely on a thickened liquid composition. Such abrasive materials, exemplified by Potter, U.S. Pat. No. 4,853,000, use a thickening agent such as flour or other components with an abrasive and solvent compositions in tarnish removal. Other paste-like silver cleaners have been well known in the art for many years.
One commercial material, Tarn-X® is an aqueous cleaner material comprising an acidified thiourea using a surfactant material and a corrosion inhibitor substance. Tarn-X® generally comprises an aqueous solution containing about 5-7 wt % thiourea, about 3-5 wt % sulfamic acid or hydroxyacetic acid and less than 1% of disodium cocoamphodiproprionate or similar detergent/inhibitor.
These prior art materials, to some degree, are effective in the removal of tarnish from such metal surfaces. Many of the materials, however, do not adequately perform the combination of roles by both removing tarnish and, at the same time, preventing tarnish return.
A substantial need exists in the art for an aqueous material that removes tarnish from metal surfaces rapidly and completely and also prevents the return of such tarnish for at least thirty days, preferably greater than 90 days, when exposed to normal environmental sources of sulfur and other tarnish forming compounds or uses. Of course, its efficacy depends entirely on the concentration of atmospheric sulfur compounds and the degree to which the metal objects are put to tarnish forming use.